Telephone system



C- E. LOMAX TELEPHONE SYSTEM Filed J an. 27. 1922 4 Sheets-Sheet 1 uenf'ur'- Clarence Elnmax July 3; 1928.

2. E. LOMAX TELEPHONE SYSTEM --Inuen'fu"r"- arenas Elnmax' Filed Jan. 2'7. 1922 Jul 3, 192s. 1,675,502

C. E. LOMAX TELEPHONE SYSTEM Filed Jan. 27, 1922 4 Sheets-Sheet 3 Elaranca Elczmax July 3, 1928. 1,675,502 C. E. LOMAX TELEPHONE SYSTEM Filed Jan. 27. 1922 Y 4 Sheets-Sheet 4 -Inuan UI' Elaranua Elumsx Patented July 3, 1928.

UNITED STATES PATENT OFFICE.

CLARENCE E. LOMAX, or CHICAGO; ILLINOIS, ASSIGNOR, BY Muslin ASSIGNMENTS, T

AUTOMATIC ELECTRIC me, or CHICAGO, rumors, A coarona'rron or DELA- WARE.

TELEPHONE SYSTEM.

Application filed January 27, 1922. Serial No. 532,063.

i The present invention relates to "telephone systems in general but is concerned more particularly with telephone systems in which calls are extended through the medium of automaticswitches; and the principalobject, briefly stated, is the provision of new and improvedtrunkcircuits for connecting branch exchanges and subeexchanges with an automatic main exchange. a y

A feature of the invention is a novel circuit arrangement whereby an opentrunk between a sub-exchangeand a main exchange isquickly discovered. Another feature of the inventionyrelates r to the prevention of a preliminary impulse when a branch exchange operator plugs into a jack in which there is terminated a trunk extending to an automatic switch.

l These objects, together with others to be pointed out lateigwill be explained fully in the course of the specification.

Referring now to thedrawings comprising Figs. 14@,inclusive, in order for them to be best understood, Figs. 1, 2, and 3 should 5 be arranged in order with the corresponding lines at the ends thereof in alignment.

V hen thus laid out, they representby means of the usual circuit diagrams a sufficient amount of apparatus for acomplete connection from a calling substation A in a sub exchange to a called substation A ina private branch exchanger Fig. 4 WllQIhSllbSti-r tuted for Fig. 1, represents a desirable modification which is used when rotary line switches are employed in the subexchange instead of the plunger type of line switch showninFigl. c

The equipment shown in Fig. 1, comprisiug the substation A, line switch C, and. holding circuit D,is assumed to be located in a subexchange; the equipment shown in Fig. 2, comprising the selectors E and E, the connector H and line switch 0 is as sumed to be located in the main exchange; and the equipment shown in Fig. 3 comprising the trunk circuit T, cord circuit 0, and the called substation A and associated equipment, is assumed to be located in a private branch exchange.

The substation A, Fig. 1, is an ordinary automatic substation having the usual talking, signalling, and switch controlling instrumentalities. The line of this substation change. nector have been omitted, only the line, re-

terminates at the subexchange in the line i switch C which, as before intimated, is a line switch of the well known plunger type.

The functions and operations of such a swltch are well known and accordingly they do not need further amplification at this time.

avoid thenecessity of having three conductors in the associated trunk X extending to the main oflice. Other functions of this circuit will be pointed out in connection with the detailed description ofits operation.

The selectorE, Fig. 2, is mechanically of the well known vertical and rotary type in lease trunkconductor which is an extension of the conductor corresponding to the one The holding circuit D is used primarily to that is connected with armature 66 of relay 61 .ofthe selector E.

The connector H is mechanically similar to. the selector E or the selector E but has a different circuit arrangement to enable it to perform the different functions assigned to 1t. This connector is of the rotary type and consequently after the calling of the last digit in a number the wipers are rotated automatically to pick out an idle trunk, in

case a call is made to a private branch ex- Most of the circuits of this con- ].ease, and back bridge relays being shown. The". circuits of this connector are assumed to be substantially the same as the cir cuits of the connector that is shown and described in the. U. S. patent to y E. Jacobsen No. 1,349,381, granted August 10, 1920, with the exception that the connector shown in the present drawings is assumed to be arranged to release when the calling party hangs up. This change may I be made in the J acobsen circuit by merely ciated with the two way trunk Y-extending between the main and the private branch exchange, is mechanically of the usual rotary' type in. Whichthe wipers have no n w mal position and move in a forward direction only. The circuits of this switch are shown complete and will be described fully hereinafter.

The trunk circuit T, Fig. 3, in which the trunk Y terminates, in the private branch exchange, is equipped with the line lamp L and the necessary cooperating apparatus for receiving incoming signals, and also with the necessary equipment for making an outgoing call. The circuits are shown complete and will be described fully hereinafter.

The remaining equipment shown in Fi 3 comprising the operators cord circuit and the manualsubstation A and associated apparatus are of a well known type and consequently do not need to be described at this time.

The automatic substation A Fig. 4, is an exact duplicate of the substation A, Fig. 1. The line switch 0, however, differs from the plunger type line switch C in that the former is of the rotary type and is identical with the rotary line switch C Fig. 2. l

The holding circuit D is very similar to the holding circuit D, Fig. 1, but differs somewhat in detail as will be pointed out in the course of the specification.

Certain of the slow-acting relays such as 14 have the copper slug on the armature end, and are both slow to energize and slow to deenergize; while other slow-acting relays such as 51 have the slug on the end opposite the ari'nature, and are only slow to deenergize. This distinction is indicated in the drawings by showing the slugs on relays such as 14 in solid black, and the slugs on relays such as 51 by section lines.

The apparatus, having been described in a more or less general manner, will now be described in connection with a detailed description of its operation. For this purpose it will be assumed that the subscriber at substation A in the subexchange desires to converse with the subscriber at substation A in the private branch exchange.

hen the receiver is removed atsubstation A, a circuit is closed over line conductors 11 and 12 for the line relay 14 of line switch C. Upon energizing, line relay 14 closes a circuit for the pull in winding. 18 which, upon energizing, operates both the plunger arm 15 and the cut oli armature 16. The former, through the medium of its plunger (not shown), forces the bank springs -33, inclusive, into engagement with the contacts 34-37, inclusive, respectively, and the latter disconnects line conductors 11 and 12 from line relay 14 and from ground, respectively. By the engagement of bank springs 30 and 33 with contacts 34 and 37, the line conductors 11 and 12 are extended by way or the upper and lower conductors of the trunk line X and arniatures 7 6 and 79 to the upper and lower windings, respectively, of line relay 60 of the selector E. Line relay 60, upon energizing, closes a circuit for release relay 61 at armature 65. Re lease. relay 61, upon energizing, prepares a circuit for switching relay 64 at armature 66, and at armature 67 opens a point in the circuit of release magnet 69 and prepares a circuit for vertical magnet 70.

Line switch C, Fig. 1, as a further result of its operation, closes a locking circuit for itself at bank springs 31 and 32 as follows: from ground by way of the supervisory relay 'tcommon to all trunks outgoing from the sub-exchange), common conductor 41, bank spring 32, contact 36, holding conductor 39, resting contact of armature 53 and said armature, release trunk conductor 38 and thence to battery by way of holding winding 17. The closing of this locking circuit, upon the operation of the line switch, obviates the necessity of having the line relay 14 slow acting enough to hold the pull in winding 18 energized until ground is fed back from switch ahead. This, however, is more or'less anincidental occurrence, the main object being to lock the line switch in its operated position when it plunges on an open trunk. This is obviously done since the line switch is now locked and will remain so until relays 51 and 52 of the trunk circuit D energize which they cannot do if one of the conductors of the trunk X is open. Since the locking circuit for the windingl'? of'the line switch C includes supervisory relay 40, the said relay energizes, thereby lighting the supervisory lamp L and operating the alarinbuzzer B.

To continue with the operation of the holding circuit 1), line relay 52, upon energizing in series with line relay 60 of the selector E in the main exchange closes a circuit for release relay 51 at armature 56. Release relay 51, upon energizing, places direct ground upon release trunk conductor 38 at armature 54, and at armature 55 closes a circuit for relay 50. Relay 50, upon energizing, disconnects release trunk conductor 38 from holding conductor 39 and consequently from the common supervisory relay 40 at armature 53. It may be said that in the normal operation, the circuit of super visory relay 40 is opened so quickly after the line switch operates that the buzzer B does not attract any attention.

The calling subscriber now operates his calling device in accordance with the lirst digit in the desired number, thereby producing a corresponding number of interruptions in the circuit of. line relay 6() of the selector E, Fi 2. Each time the line relay deenergizes in response to one of these interruptions it closes a circuit for vertical magnet as follows: from ground by way of the resting contact of armature 78 and ill) said armature, armature and its resting contact, armature 67 and its Working contact, relay 62,and vertical magnet 0 to battery. By the operation of vertical magnet 70, the switch wipers 8082, inclusive, areraised step by step until they stand opposite the desired level of bank contacts.

Relay 62 is energized in series with vertical magnet 70 andbeing slow acting retains its armature attracted throughout the vertical movement, and,upon the closureof ofi normal contacts 73, which occurs at the first vertical step,closes a circuit for stepping relay 63. Stepping relay 63, upon energizing, closes a locking circuit for itself at armature 72, and at armature 74 prepares magnet 83 at armature 74, whereupon the said rotary magnet deenergizes andcloses its interrupter contacts 75 again.

From this point the operation depends upon whether the trunk terminating at the first set of bank contacts is busy or idle. If it is idle switching relay 64 energizes immediately, but if it is busy switchin relay 64 is short circuited and stepping re ay 63, upon energizing, closes a circuit for rotary magnet 83 at armature 74, whereupon the wipers are advanced into engagement with the second set of contacts. This alternate operation ofstepping relay 63 and rotary magnet 83 continues until an idle set of corn tacts is reached. hen this occurs switching relay 64, being no longer shortcircuited, energizes in series with stepping relay 63.

Stepping relay 63, however, does not 'energize at this time owing to the high resistance of switching relay 64. Switching relay 64, upon energizing, opensthe testing circuit and prepares the holding circuit at armature 77; disconnects ground from armature 65 of line relay 60 at armature 78; and at armatures 7 6 and 79 disconnects the incoming trunk conductors from the windings of line relay 60 and extends them by way of lays of the selectors E and E.

them in search of an idletrunk. Assuming the trunk selected to be the one extending to the connector H, the line and release relays 86 and 87 energize, thereby preparing the switch for operation and closing the usual holding circuit for the switching re p In response to the calling of the next two digits in the desired number the connector H raises its wipers to the desired level and thenrotates them into engagement with the first trunk in thedesired group, after which the wipers are rotated automatically in search of an idle trunk. Assuming the trunk Y tobe the first one. foundto be idle, wiper .101 places ground upon test contact 102, when the wipers (of the connector arrive upon the set of contacts in which the trunk Y terminates, thereby. closing a, circuit'for the combined switching and cut off relay 104 of the line switch C 1n series with stepping magnet 106. Switching relay 104 energizes over this circuit but stepping magnet 106 does not energize on account of the high resistance of switching relay104. Owing to a mechanical interlocking device between the armature of line relay 1'05 and the armature of relay 104, thelatter relay is energized only about half wayat this time with the result that armatures 109-111, inclusive, are separated from their resting contacts but are not moved into engagement with their working contacts.

In the connector H, the switching relay (not shown) upon energizing, connects up the ringing circuit, thereby projecting ringing current outover the called trunk.

In the trunk circuit T, Fig. 3. the high wound polarized relay 126 which isnormally energized in series with line relay 105 of the line switch C, is now energized in the reverse direction, owing to the normal conductors being reversed at the banks of the connector H in the main exchange, and consequently springs 131 and 132 are allowed to come into engagement with their resting contacts. It may be said at this point that,

owing to the high resistance of polarized relay 126, the line relay of theline switch C is not able to energize in series with it when thetrunk is idle and the ring out off relay (not shown). of the connector His not able to energize in series with the said polarized relay 126when the trunk is seized. I

To continuewith the operation, spring 131, upon coming into engagement with its resting contact. closes the following circuit for the linelampL': from ground by way of said lamp L, spring 131 and its resting contact, andthe resting contact of armature 130 and said armature to battery. The line lamp L accordingly becomes lighted.

The operator, upon noting" the lighted condition of the lamp L, inserts the answering plug of an idle cord circuit, the plug P of the cord circuit 0 tor example, into the jack J, thereby closing the following circuit for relay 125: from ground by way of the upper winding of relay 125, resting contact of spring 132 and said spring, lower right hand winding of the repeating coil, ring cont-act of the jack J, ring contact of the plug P, supervisory relay 140, and the lower left hand winding of the associated repeating coil to battery. Relays 125 and 140 energize in series over this circuit and the latter, upon energizing, places a shunt around the supervisory lamp L thereby preventing the saidlamp from becoming lighted at this time. Relay 125. upon ener gizing. opens the circuit of the line lamp L and closes a locking circuit for its lower winding at armature .130as follows: from ground by way of the upper left hand winding of the repeating coil in the cord circuit 0, tip contact of the plug P, tip spring of the jack J, upper right hand winding of the repeating coil associated with the said jack J, lower, winding of relay 125, and the working contact of armature 130 and said armature to battery. As a further result of the energization of relay the resistance 129 is placed in multiple with the polarized relay 126 at armature 128. hen this occurs the resistance of the bridge across the conductors ot the trunk line Y is lowered sufliciently to allow the ring cut ofl' relay (not shown) of the connector H to energize, thereby connecting the line wipers of the said connector with the upper and lower windings of bacli bridge relay, 85, as shown by the dotted conductors. Back bridge relay 85, upon energizing, reverses the incoming trunk conductors as regards their connection with the windings 0t line relay 86.

. The consequent reversal current flow has no particular function in the present,connection but is used. at times to give supervision to an operator and for other similar purposes.

The operator now throws her key K, which is a combined listening and ringing key. into listening position, thereby connecting up her head set so as to enable her to converse with the calling subscriber and,

assuming that she learns that the calling subscriber desires to be connected with the subscriber at substation A. she inserts the calling plug P of the cord circuit 0 into the jack J, thereby closing a circuit which includes cut off relay 142 and supervisory hen this occurs the said supervisory lamp L becomes lighted and cut off relay 142, upon energizing, clears the called line.

The operator now rings the called subscriber by throwing her ringing 'key into ringing position momentarily.

lVhen the called subscriber responds by removing his receiver, a circuit is closed for supervisory relay 141 and the said relay, upon energizing, places a shunt around the lamp L, whereupon the said lamp becomes extinguished, thereby giving the operator answering supervision.

The calling and called subscribers may now converse with each other as desired, the talking circuits being outlined by the heavy conductors, and when the conversation is completed they both replace their receivers.

\Vhen the receiver is replaced at the called substation. the circuit of supervisory relay 141 is opened and the said relay, upon deenergizing, removes the shunt from around supervisory lamp L whereupon the said lamp becomes lighted.

Vhen the receiver is replaced at substation A the circuit of line relay S6 of the connector H, Fig. 2, and of line relay 52 of the holding circuit D, Fig. 1. is opened. In the connector H, line relay 86, upon deenergizing,opens the circuit of release relay 8? and the said release relay 87, upon deencrgizing, removes ground from the release trunk conductor and closes the usual release magnet circuit, whereupon the switch re stored to normal in the usual manner, In response to the removal of ground from the release trunk conductor, the selectors E and E are restored to normal position in the usual manner by the operation of their respcctive release magnets. The circuit of re lease magnet (39 of the selector IE, it may be pointed out, includes the resting contact of armature 78 and said armature, armature 65 and its resting contact, armature 67 and its resting contact, and off normal con tacts 68.

In the subexchange, line relay 52, upon deenergizing. opens the circuit of release relay 51 at armature 56. Release relay 51, upon deenergizing, opens the circuit of relay 50 at armature 55 but the said relay 50, being slow acting, retains its armature 53 attracted for an interval. As a further result of the deenergization of release relay 51, ground is removed from release trunk conductor 38 at armature 54, thereby opening the circuit of the holding winding 17 0f the line switch C. When this occurs the plunger arm is withdrawn and springs ant 33 move out of engagement with contacts 3 1 and 37. A moment later. slow acting relay of the holding circuit D deenergizes, thereby connecting conductors 3S and 39 to gether again at armature 53.

As a result ofthe release of the connector H, the windings of back bridge relay are disconnected from the trunk Y and the circuitof relay 104 of'the line switch C is opened. Relay 104, upon deenergiziug, connects the winding of line relay 105 and ground to the upper and lower conductors of the trunk Y respectively, at armatures 109 and i111. thereby reversing the current flow infthe conductors of the trunk Y back to normal. An incidental operation which occurs at this time is thatline relay 105 of the line switch C? energizes, thereby causing the line switch to search for-an idle trunk in the usual manner.

The operator, upon noting the lighted condition of the lamps L and L pulls down the connection. When the plug is removed from the jack J, cut oil' relay 142 deenergizes and supervisory lamp L becomes extinguished.

, When the plug is removed from the jack J the supervisory lamp L becomes extinguished, and relay 125 deenergizes, as result of the openlng or its circuit, thereby preparing a circuit for the line lamp L at armature 130, and disconnecting resistance 129 from in multiple with the high .wound polarized relay 126 at armature 128. When this occurs, the trunk seized by the line Switch C Fig. 2,'is freed. The entire connection is now released and the apparatus used therein is restored to normal.

It will be assumed now that the subscriber. at substation A in the private branch exchange desires to converse with some subscriber in the main exchange. When the receiver is removed at substation A, a circuit is closed over the associated line conductors for line relay 143, andthe said relay,

upon energizing, closes a circuit for line lamp L. I i p The operator, upon noting the lighted condition of the lamp L inserts the answering plug of an idle cord circuit into the jack J, the answering plug P of the cord circuit 0 for example, thereby closing the usual circuit for supervisory lamp L and cut off relay 142 in series, whereupon the line relay is cut oil and the lamp L becomes extinguished. a

After listening in and ascertaining the desired number, the operator inserts the calling plug P into the jack J, thereb closing a circuit for the supervisory lamp It will be noted that the plugging in of the operator into the jack J doesnot close any bridge across the conductors of the trunk line Y. The closing of the bridge is. de-

layed, until the operation ofthe calling device key K has takenplace, so as to prevent any possibility of the operator causing preliminary impulses in i the automatic switches by plugging in carelessly.

The operator now operates the calling device key K, thereb disconnecting the conductors of the trun 'line Yfrom thefwindings ofthe repeating coil and bridging the calling. device S across the said conductors. When this occurs line relay 105 ofthe line switch C in the main exchange, upon energizing,"plac'es ground upon private normal conductor 103'at armature 108, thereby clos ing a circuit for switching relay 104 and step ping magnet 106 in series, and at armature 107 connects test wiper 113 to the circuit at a point between switching relay 104. and

stepping magnet 106. The lineswitch C now operates in the well known manner to select an, idle trunk, beingadvanced step by step by the buzzer like action of stepping magnet 106. Whenan idle trunk is reached, switching relay 104, being no longer short circuited, energizes in series with stepping magnet 106; opens the test circuit and prepares the holding circuit at armature 110 and, at armatures 109 and 111, disconnects the incoming trunk conductors from the winding of-line relay 105 and ground and extends them by way of wipers 112 and 114,.

and the bank contacts upon which these wipers are standing, to the line relay of an idle first selector, which selector will be similar to the selector E. The line and release relays of the seized selector now operate in the usual manner to prepare the switch for operation and to place ground upon theasso ciated release trunk conductor, thereby establishing the usualholding circuit for the line switch C In the private branch exchange, as a result ofthe windings of the repeating coil being disconnected from the conductors of the trunk. line Y, polarized'relay 126 deenergizes, thereby. closing a circuit for the upper windingof relay 125 at spring 132 as follows: from ground by way of the upper winding of relay 125, resting contact of s ring 132 and said spring, lower right han ,winding of the associated repeating coil, ring spring of the jack J, ring contact of the plug P, and thence to battery by way of supervisory relay 141. Supervisory relay 1411, upon energizing, places a shunt around supervisory lamp L whereupon the said lampbecomes extinguished. Relay 125, upon energizing,closes a locking circuit for its relay 125, and the working contact of armature and saidarmature to battery. As a further result of the energization of relay 125, it places resistance 129 in shunt of the high wound relay 126 at armature 128.

The 0 erator now manipulates hercalling device in accordance with the various digits of the desired number, thereby causing the connection to be extended to the v desired line and causing the called subscribers bell to be rung in the usual manner.

After having finished dialling the number, the operator restores her key K to normal, thereby shifting the bridge across the conductors of the trunk line Y so as to include the left hand windings of the associated repeating coil, armature 128 and its working contact, and resistance 129, in multiple with polarized relay 126. Polarized relay 126, upon energizing, in series with the line relay of the 'connector (not shown) 'in the main exchange, opens the circuit of the upper winding of relay 125 and of supervisory relay 141 at spring 132, whereupon supervisory relay 141 deenei'gizes and the associated supervisory lamp L becomes lighted. Relay 125 in the trunk circuit T, however, does not deenergize, being maintained energized over its locking circuit which includes its lower winding and armature 130.

\Vhenthe called subscriber responds, the consequent reversal of current flow in the established connection attracts the'arniature 127 of polarized relay 126 in the opposite direction, thereby allowing springs 131 and 132 to close. Spring 132, upon coming into engagement with its associated contacts closes a circuit for the upper windin of relay 125 and supervisory relay 141 ot the cord circuit 0 in series. Supervisory relay 141, upon energizing, laces a shunt around the supervisory lamp whereupon the said lamp becomes extinguished, thereby giving the operator answering supervision. The two subscribers now converse with each other in the usualmanner.

When the subscribers have finished conversing they replace their receivers whereupon the supervisory lamps L and L in the cord circuit 0 become lighted, the lamp L becoming lighted in response to the falling back of supervisory relay 140 which occurs when the receiver is replaced at the substation A, and the supervisory lamp L becoming lighted in response to the deenergization of supervisory relay 141 which occurs when its circuit is open at spring 132 of polarized relay 126, whenthe current flow is reversed back to normal, upon the han ing up of the called subscriber.

Q ien the operator pulls downthe connection the equipment is restored to normal in the usual manner. I

Referring now to Fig. 4, the operation of the apparatus disclosed therein will be explained. This apparatus corresponds to the ap aratus showin in Fig. 1 and the chief di erence is that the line switch C has been replaced by the rotary type line switch C. The circuits of the holding circuit D have been modified slightly so as to operate satisfactorily. in this connection. The trunk line X it will be understood, extends to the main exchange and terminates'there in a selector similar to selector E, Fig. 2.

Assuming now that the subscriber at substation A makes a call, a circuit is closed over line conductors 211 and 212 for line relay 215 of line switch C. Upon the energization of line relay 215, the line switch C operates in the usual manner to select an idle trunk. Assumin that the trunk comprising conductors 230-237, inclusive, is the first onefound to be idle, switching relay214 of the line switch C, upon energizing when said trunk is reached, opens the test circuit and prepares the holding circuit at armature 217, thereby placing ground upon release trunk conductor 236 by way of wiper 223 and bank contact 226. Vi hen this occurs relay 238 energizes over a circuit which includes release trunk conductor 236 and armature 243 and its resting contact; and, upon so doing, places a multiple ground upon release trunk conductor 236 at armature 241, thereby locking the line switch C in on the seized trunk. As a further result of the energization of relay 238, it places ground upon the common alarm conductor 245 at armature 242. thereby closing a circuit for alarm relay 246;and the said relay, upon energizing, closes a circuit for lamp L at armature 247. and for the buzzer B at ar mature 248.

As a further result of the energization of switching relay 214 of the line switch C, it extends the line conductors 211 and 212 by way of armatures 216 and 218 and their working contacts, wipers 222 and 224, bank contacts 225 and 227, conductors 235 and 237, and'line relay 240 to the conductors of the trunk line X. Assuming now that one of the conductors of the trunk line X is open line relay 240 does not energize and the line switch C remains locked on the seized trunk as described; and the attendant, upon hearing the buzzer B and observing the lamp L takes the necessary steps to clear the trouble.

Assuming now that the conductors of the trunk line X. are continuous, line relay 240 of the holding circuit D is energized in series with the line relay of the selector to which the trunk line X extends and closes a circuit for release relay 239 at armature 244. Release relay 239, upon energizing, opens the circuit ofrelay 238 and places a multiple ground upon the release trunk conductor 236 at armature 243. Relay 238, upon deenergizing, opens th alarm circuit at armature 242, and removes ground from release trunk conductor 236 at armature 241, leaving the said conductor grounded only at armature 243 of release relay 239.

The calling subscriber may now go ahead and call the remaining digits in the desired number in the usual manner; and, when the number has been dialled and'the conversa- Ill! tion has been completed the connection is released in the usual manner, upon the receiver being replaced at the calling substation.

The various features of the invention, having been described and ascertained in the foregoing, will now be pointed out specifically in the appended claims.

W'hat is claimed is: i

1. In a telephone system, an automatic trunk selecting switch having movable contacts, trunk lines accessible to said switch and terminating in'fixed contacts, means for operating said switch to select and connect with an idle trunk, a lockin circuit for holding said switch in operated position including a movable switch contact and a fixed contact associated with the selected trunk, means operable over a non-talking conductor of said trunk for closing said locking circuit, a holding circuit for said switch closed after said locking circuit, and means for opening said locking circuit when the holding circuit is closed. a I

2. In a telephone system, an automatic trunk selecting switch having movable contacts, trunk lines accessible to said switch and terminating in fixed contacts, means for operating said switch to selectand connect with an idle trunk, a locking circuit for holding said switch in operated position including a movable switch contact and a fixed contact associated with the selected trunk,

and means controlled over and individual to the selected trunk for opening said look ing circuit while maintaining the switch in operated position.

3. In a telephone system, an automatic trunk selecting switch having movable contacts, trunk lines accessible to said switch and terminating in fixed contacts, means for operating said switch to select and connect with an idle trunk, a locking circuit for holding said switch in operated position ineluding a movable switch contact anda fixed contact associated with the selected trunk,

means controlled over and individual to the selected trunk for closing a holding circuit to retain said switch in operated position, and means for opening said locking circuit.

4. In a telephone system, an automatic trunk selecting switch and trunk lines ac cessible thereto, means for operating said switch to select and connect with an idle trunk, a locking circuit for retaining said switch in operated position established responsive to the completion of the connection with the selected trunk, said circuit being established whetherthe selected trunk is in operative condition or not, and means operating automatically contingent on the operative condition of the selected trunk for opening said locking circuit.

5 In a telephone system, a subexchange trunk line including two talking conductors and a local third conductor, line switches in the sub-exchange having access to said trunk line, means ior operating one of said switches to connect with said trunk, relay equipment in said trunk line adapted to complete a holding circuit for the operated line switch, and a locking circuit including the third conductor of said trunk established when the trunk is connected with for retaining the switch in operated position until the said holding circuit is established.

6. In a telephone system, a trunk hunting switch provided with a relay, a trunk circuit accessible to said switch including a second relay, means controlled by a calling subscriber through'the medium of the first named relay for operating said switch to seize said trunk, and means independent of thecalling subscribers line for energizing said second relay responsive to said seizure for locking said switch on said trunk. y

7. In a telephone system, a trunk hunting switch provided with a relay, a trunk circuit accessible to said switch including a second relay, means controlled by a calling subscriber through the medium of the first named relay foroperating said switch to seize said trunk, means for energizing said second relay responsive to said seizure for locking said switch on said trunk, said sec- 0nd relay being energized whether the trunk is in workin condition or not, a third relay associated with said trunk operated responsive to said seizure only in case said trunk is in working condition, and means, controlled by said thirdrelay for unlocking said switch to place the same under control of the calling subscriber.

8. In a telephone system, a trunk line, a line switch operable to select and connect with said trunk line, a relay in said trunk line initially energized from said switch, a locking circuit for said relay, and a holding circuit for said switch controlled by said relay.

9. In a telephone system, a trunk line, a line switch operable to select and connect with said trunk line, a selfdocking relay in said trunk line initially energized from said switch over a non-talking conductor of said trunk line, and a holding circuit for said switch controlled by said relay.

In witness whereof, I hereunto subscribe my name this 23d day of January, A. D., 1922.

i CLARENCE E. LOMAX. 

